Tap#install: better TapRemoteMismatchError check

* remote check requires `git` installed.
* Do not perform check if user does not passing remote explicitly.

Fixes #108

Library/Homebrew/tap.rb

@@ -188,7 +188,6 @@ class Tap
     requested_remote = options[:clone_target] || "https://github.com/#{user}/homebrew-#{repo}"
 
     if installed?
-      raise TapRemoteMismatchError.new(name, @remote, requested_remote) unless remote == requested_remote
       raise TapAlreadyTappedError, name unless full_clone
       raise TapAlreadyUnshallowError, name unless shallow?
     end
@@ -197,6 +196,10 @@ class Tap
     Utils.ensure_git_installed!
 
     if installed?
+      if options[:clone_target] && requested_remote != remote
+        raise TapRemoteMismatchError.new(name, @remote, requested_remote)
+      end
+
       ohai "Unshallowing #{name}" unless quiet
       args = %W[fetch --unshallow]
       args << "-q" if quiet

Library/Homebrew/test/test_tap.rb

@@ -181,9 +181,10 @@ class TapTest < Homebrew::TestCase
   def test_install_tap_remote_mismatch_error
     setup_git_repo
     already_tapped_tap = Tap.new("Homebrew", "foo")
+    touch @tap.path/".git/shallow"
     assert_equal true, already_tapped_tap.installed?
     wrong_remote = "#{@tap.remote}-oops"
-    assert_raises(TapRemoteMismatchError) { already_tapped_tap.install :clone_target => wrong_remote }
+    assert_raises(TapRemoteMismatchError) { already_tapped_tap.install :clone_target => wrong_remote, :full_clone => true }
   end
 
   def test_install_tap_already_unshallow_error